This application is based on French Patent Application No. 01 11 638 filed Sep. 5, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The present invention relates to semiconductor optical components, and more particularly to a semiconductor optical component comprising a p-type doped semiconductor material confinement layer, and to a method of fabricating this type of component.
2. Description of the Prior Art
A semiconductor optical component, such as a semiconductor optical amplifier, is often used in prior art pumping sources rated at a few watts for amplifying wavelength division multiplex (WDM) optical signals by means of an optical fiber, in particular for Raman amplification of such signals. The operation of this kind of amplifier is based on the use of an active layer which, once supplied with current, amplifies an injected optical wave, generally at a wavelength from 0.8 xcexcm to 1.6 xcexcm, as it propagates.
A semiconductor optical component has a confinement layer disposed on top of the active layer and contributing to guiding the wave propagating in the component. The confinement layer is a layer of a semiconductor material containing electron acceptors creating positive free carriers known as holes. The holes then account for the greater part of the electrical conduction: the doping is referred to as p-type doping, which among other things yields a semiconductor confinement layer of lower electrical resistance than the intrinsic resistance of the semiconductor, which in particular avoids overheating of the component and ensures good injection of current into the active layer. However, this kind of doping introduces losses in the optical component, for example through photoabsorption. This is because the holes are able to absorb photons emitted by the active layer, thus limiting the optical power available from the component.
The confinement layer defines a plane parallel to the active layer. In modern components, the concentration of p-type dopants is uniform in this plane. To fix the value of the concentration, a compromise is therefore arrived at to obtain the lowest possible resistance and at the same time the lowest possible overall losses.
The probability of photoabsorption increases with the optical power, i.e. with the number of photons. Accordingly, for a given concentration of p-type dopants, the losses generated are greater in some regions of the optical component. Similarly, the intrinsic resistance of the confinement layer can vary locally, for example if the thickness or the width of the layer varies.
An object of the present invention is therefore to provide a semiconductor optical component comprising a p-type doped semiconductor confinement layer having opto-electrical properties that are optimized in each region in order to improve the performance of the component.
To this end, the present invention proposes a semiconductor optical component including a semiconductor material confinement layer containing acceptor dopants such that the doping is p-type doping, in which component the confinement layer is deposited on another semiconductor layer and defines a plane parallel to the other semiconductor layer and the p-type doping concentration of the confinement layer has at least one gradient significantly different from zero in one direction in the plane.
The gradient(s) are determined as a function of the local electro-optical properties required in the semiconductor optical component according to the invention.
The direction can advantageously be the light propagation axis, the optical power varying along that axis.
According to the invention, the sign of the gradient can be negative, which is advantageous in a region necessitating minimum optical losses.
In one embodiment of the invention, the semiconductor material further contains donors whose concentration is lower than the concentration of the acceptors over the whole of the direction so that the doping remains of the p-type.
In this configuration, the p-type doping concentration corresponds to the difference between the acceptor concentration and the donor concentration. The doping gradient can thus be obtained from a donor concentration gradient.
The acceptors are preferably chosen from zinc, magnesium and cadmium and the donors are preferably chosen from silicon and sulfur.
According to the invention, the semiconductor material can be chosen from alloys based on materials from column(s) 111 and/or V of the periodic table of the elements, such as alloys based on InP and GaAs.
In one preferred embodiment of the invention, the component is a tapered guide semiconductor optical amplifier including a tapered optical guide having a length L along the light propagation axis disposed between an entry face and an exit face, the tapered guide including a cross section region having a constant surface area, referred to as the constant section region, for monomode propagation of light, the constant section region leading to a cross section region having an increasing surface area, referred to as the increasing section region, in order to reduce the optical power density of the semiconductor optical amplifier. Also, the confinement layer is part of the optical guide and the gradient is negative and is in the increasing section region along the propagation axis.
The surface area of the increasing section region is greater than that of the constant section region so that the resistance in the increasing section region is less than that in the constant section region. Moreover, as the optical power increases from the entry face to the exit face, the optical losses are liable to be greater in the increasing section region. It is therefore advantageous to reduce the p-type doping concentration in that region.
In one configuration of this latter embodiment, the gradient is situated over the whole of the increasing section region to optimize the performance of the component according to the invention.
To increase the optical power available from the component according to the invention, the p-type doping concentration can advantageously be at a minimum at the exit face.
Furthermore, in this latter embodiment, the minimum concentration is from 1017 cmxe2x88x923 to 1018 cmxe2x88x923 and is preferably substantially equal to 3xc3x971017 cmxe2x88x923.
The invention also proposes a method of fabricating a semiconductor optical component according to the invention, the method including the following steps:
a masking step by depositing a silica type dielectric material masking layer onto another semiconductor layer,
a step of partial elimination of the masking layer to create an opening exposing the other layer, and
a step of depositing the confinement layer, in which the confinement layer is deposited selectively in the opening.
The confinement layer in accordance with the invention is selectively deposited over the opening using the selective area growth (SAG) technique described in the paper by A. M. Jones et al. xe2x80x9cGrowth, Characterization, and Modeling of Ternary InGaAsxe2x80x94GaAs, Quantum Wells by Selective Area Metalorganic Chemical Vapor Depositionxe2x80x9d, Journal of Electronic Materials, Vol. 24, No 11, 1995, pages 1631-1636. The SAG technique is based on the fact that GaAs or InP type materials are not deposited on a silica oxide mask and therefore diffuse toward the semiconductor substrate.
The method according to the invention can further include a step of totally suppressing the masking layer after the step of depositing the confinement layer.
The method of fabricating a component in accordance with the invention can include a step of partly suppressing the confinement layer after the total suppression step.
The features and objects of the present invention will emerge from the following detailed description, which is given with reference to the accompanying drawings, which are provided by way of illustrative and non-limiting example.